1. Field of the invention
The present invention relates to a fuel supply control apparatus for an internal combustion engine having an air/fuel (A/F) ratio feed-back control with a learning function, especially to the improvement of the efficiency of the learning operation in such a fuel supply control apparatus.
2. Description of the related art
As is well known, there has been widely employed an A/F ratio feed-back control, using an oxygen sensor, for controlling an A/F ratio of a fuel mixture supplied to an internal combustion engine. Further, recently, use of a so called learning function in such an A/F ratio control is receiving increasing attention, in order to improve the response in the A/F ratio control.
In an A/F ratio feed-back control system having a learning function, an actual A/F ratio of a fuel mixture is detected by a known oxygen sensor and an A/F ratio of a fuel mixture is controlled so as to make the actual value thereof follow a reference value, which is usually set at the stoichiometric A/F ratio. Data concerning a deviation between the actual value and the reference value are stored as correction values in areas of a correction map, each area corresponding to a particular operational condition, which is defined by, for example, the rotational speed of the engine and the load thereof.
When the engine encounters a certain operational condition during operation thereof, data stored in an area of the map corresponding to the certain operational condition is read out and an A/F ratio determined, for example, by an suction air amount and a rotational speed of the engine, is corrected on the basis of a correction value, i.e., the data read out from the map. With this, a fuel mixture can be always maintained at an appropriate A/F ratio, taking account of characteristics of a particular engine and operating circumstances thereof.
In the A/F ratio feed-back control system as mentioned above, as is well known, it is necessary to renew correction values stored in the map in accordance with changes in operating conditions of the engine. For example, even if the engine is operated at the same rotational speed and the same load, it is necessary to change the A/F ratio of a fuel mixture in accordance with the elevation of the road on which the automobile travels in order to make an actual A/F ratio follow the stoichiometric value more accurately.
When an automobile travels in a mountain district, for example, there frequently occurs cases where the elevation of the road changes widely and accordingly the atmospheric pressure changes. Namely, the operating conditions of an engine often changes.
In such cases, the learning operation as mentioned above and the renewal of data stored in a correction map must be repeated, every time the automobile encounters new traveling circumstances. According to circumstances, the renewal of data of the map can not be executed fast enough to obtain an appropriate correction value in time, and therefore the actual A/F ratio is made worse.
To improve this, there has been proposed a method of renewing data of a correction map as disclosed, for example, in the laid-open Japanese patent application No. JPA-59/25055 (published on Feb. 8, 1984).
According to this, there are provided two memories, in which a first one of the memories is equivalent to a correction map as described above and stores correction values in response to operational conditions of an engine. Data stored therein is subject to the renewal by means of a learning operation in the same manner as mentioned above.
A second memory stores data, which is obtained by averaging differences between a predetermined value and the correction values stored in selected areas, including at least some areas neighboring or surrounding a corresponding area, of the correction map.
When a final amount of fuel to be supplied to the engine is determined, data stored in an area of the map corresponding to the operational condition of the engine at that time is at first read out from the first memory. Then, the read-out data is combined with data stored in the second memory, whereby a correction value used for the determination of the final fuel supply amount is formed.
As will be understood from the foregoing, a concept underlying the prior art described above is as follows; when a factor, for which a correction value must be renewed, is extracted as the result of operation of the engine in a particular operational condition, the influence of the factor is brought on the operation of the engine in neighboring operational conditions.
Accordingly, although, when the operation of an engine changes to a neighboring operational condition, a correction value stored in an area of a correction map corresponding to the neighboring operational condition is read out, it is already corrected to a certain extent during the operation of the engine in the previous operational condition. Therefore, an appropriate A/F ratio can be determined quickly and therefore the response in the A/F ratio feed-back control is improved considerably.
However, the prior art as described above does not have a sufficient affect in the point of view of efficiently renewing data stored in the second memory, because the calculation for obtaining the average of differences of correction values stored in the selected areas of the correction map and the predetermined value must be carried out frequently.